This invention relates to implantation of boron into silicon and more particularly to a boron implantation method of making PN junctions on &lt;100&gt; N-type silicon crystal faces that permits anneal in a strongly oxidizing atmosphere to passivate the PN junction.
An N-type monocrystalline silicon surface can be doped to P-type conductivity by exposing the silicon to a beam of boron ions. Ordinarily, one uses an ion beam of the boron isotope .sup.11 B, the boron isotope having an atomic weight of eleven. After implanting a prescribed dose of the boron ions, the silicon is annealed. In annealing, crystal defects caused by the implantation are repaired, and the implanted boron atoms are moved to substitutional sites in the crystal lattice. In addition, annealing produces boron diffusion in the lattice. It can be done in reducing inert or oxidizing atmospheres. If done in a strongly oxidizing, i.e., moist oxygen atmosphere, it will also produce a passivating silicone dioxide coating over the silicon surface. If an island-like surface area of the N-type monocrystalline silicon was selectively implanted with boron, a surface intersecting PN junction will result. The silicon dioxide coating will quite effectively passivate this junction. Hence, where junction passivation is desired, it is desired to anneal in a strongly oxidizing atmosphere.
Implantation of .sup.11 B in an island-like area on a &lt;111&gt; N-type silicon surface, followed by annealing in any of the usual atmospheres, including moist oxygen, will result in low leakage PN junctions. We have recognized that implantations of .sup.11 B can be used to make low leakage PN junctions in &lt;100&gt; N-type silicon surfaces too, but only if the anneal atmosphere is not strongly oxidizing. Lowest leakage PN junctions are attained by .sup.11 B implantations in &lt;100&gt; N-type silicon when the anneal atmosphere is neutral, as for example nitrogen or an inert gas, or only midly oxidizing, as for example dry oxygen. If one wanted to passivate this PN junction on &lt;100&gt; silicon during anneal a strongly oxidizing, for example moist oxygen atmosphere might be used. However, as mentioned annealing the &lt;100&gt; silicon in strongly oxidizing atmosphere produced PN junctions with higher leakage. Thus, one had to be content with higher leakage PN junctions in boron implanted &lt;100&gt; N-type silicon, if moist oxygen passivation during anneal was desired. We have now found an implantation technique that permits such passivation without the higher PN junction leakage.